56 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



After the exclusion of the water, and when the dam had 

 been sufficiently strutted against the old wall, a row of piles 

 25 feet long by 12 inches square was driven on the land side 

 about 32 feet from the inner row of piling of the dam, and 

 spaced to correspond with the main piles ; between these gauge- 

 piles a row of sheet-piling was driven, extending from the surface 

 of the roadway to below the level of the foundation of the 

 old wall. 



As the masonry of the old wall and the struts against it 

 were removed, whole-timber shores 30 feet long were inserted, 

 resting against the land row of whole piles and the dam. A 



Ht'ah Water Eauinoct!al< Spring Tides 



Hiah Water Ordinary Sorina Tides 



35 Ft. below O.D.S. 



FIG. 43. 



lower row of continuous sheet-piling, 24 to 26 feet long, 12 

 inches by 6 inches, was then driven along the land side. The 

 excavation and shoring was then continued to the bottom. 



The weak point in this dam appears to have been that the 

 walings and shores were placed too far apart, and that the 

 fastenings generally were too weak; and although it stood 

 and answered its purpose, it must have been strained veiy 

 nearly to its ultimate powers of endurance. 



Dam across the Entrance to the Low-water Basin at Birkenhead. 1 

 This dam (Fig. 43) was on plan a segment of a circle with 

 1 M.P.I.C.E., vol. li. p. 146. 



